How do you convert heat energy to mechanical?

In some refrigeration systems, mechanical energy is converted to heat energy by mechanically compressing a refrigerant, by means of a compressor; and since refrigerants have a fixed pressure-temperature relationship, much like saturated steam, altering the pressure of a refrigerant correspondingly alters its temperature. I've read of other methods utilizing friction, which is itself mechanical energy in the more pure and direct sense rather than the operation of a mechanical device which itself is utilizing electrical energy, but the compressor example is the only practical real-world application of this nature in common, everyday use that I can think of.